Early studies tested mechanisms for accessing street populations of heroin users and assessing the nature and extent of their drug use and criminality. Subsequent studies targeted a variety of crime-involved heroin and cocaine users, including both women and men, serious delinquents, adolescent and adult crack users, and cocaine users in treatment and on the street. Major findings included the low risk of arrest for income- generating crimes committed by heroin users and the prevalence of HIV risk behaviors among both serious delinquents and female crack users. Analyses consistently showed the critical importance of sample characteristics in research on drug use, including age, cohort, and street versus treatment status. Drug use was not the primary cause of criminality as most drug users had already embarked on criminal careers prior to the onset of drug use. 52 references and 8 tables